GRB 210308A: MITSuME Okayama optical upper limits

We observed the field of GRB 210308A (D'Elia et al.,GCN Circular #29619) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope of Okayama Astrophysical Observatory, Okayama, Japan. The observation with a series of 60 sec exposures started at 2021-03-08 11:03:59 UT.(4.5 hours after Swift BAT trigger) We stacked the images with good conditions. We did not detect the optical afterglow reported previously (D'Elia et al.,GCN Circular #29619) in all three bands. We obtained the 5-sigma limits of the stacked images as follows. T0+[hour] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 4.5 11:51:53 4440 g'>17.6, Rc>17.8, Ic>17.8 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time We used the PS1 catalog for flux calibration. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire).